Determination of Carbohydrates in Foodstuffs by Ion Chromatography with Pulsed Amperometric Detection

Alfred Steinbach and Andrea Wille, Metrohm AG

This article describes a straightforward ion chromatographic method that uses isocratic elution and pulsed amperometric detection (PAD) to sensitively determine water-soluble polyols and sugar alcohols as well as mono-, di- and oligosaccharides in essential and nonessential foodstuffs. While carbohydrate determination of most foodstuffs requires only minimal sample pretreatment such as dilution and filtration, samples with interfering matrices such as protein-containing dairy products have to be dialyzed prior to injection.

By means of two applications the paper demonstrates the potential of ion chromatography followed by pulsed amperometric detection in foodstuffs.

Instrumentation

Chromatographic equipment:

871 Advanced Bioscan

838 Advanced IC Sample Processor

833 Advanced IC Liquid Handling Pump Unit

833 Advanced IC Liquid Handling Dialysis Unit

818 Advanced IC Pump

Instrument control, data acquisition, and processing were performed by Metrodata IC Net software (Metrohm AG).

Example 1: Carbohydrate determination in malt extract

Viscous or dried malt extracts are obtained from germinated barley and contain naturally present enzymes, particularly amylase, which convert starch into water-extractable sugars. Malt extracts excel in their high physiological and nutritional values. Malt is added as a nutritional supplement to the diets of infants and elderly persons. Furthermore, it is a very important intermediate ingredient in infant and pet foods, variety and party breads, instant coffee, beverages, ice creams, pharmaceuticals, etc.

Glucose, fructose and sucrose are primarily responsible for the sweetness. Since the latter two sugars were not present at detectable concentrations (Figure 1), malt extracts are only perceived half as sweet as mainly sucrose-containing products. Apart from glucose and maltose the investigated malt extract contains several maltooligosaccharides.

Figure 1

The observed saccharide profile of the investigated sample corresponds to the general malt composition.

Example 2: Carbohydrate content in dairy products

In contrast to the straightforward sample preparation of soluble sugar constituents in malt extracts presented above, the analysis of protein-containing samples such as dairy products can pose severe problems. Precipitation of the proteins fouls the column und ultimately destroys it. This problem can be overcome by using stopped-flow dialysis. This technique is based on the selective diffusion of molecules or ions from one liquid (donor or sample solution) to another (acceptor solution) via a membrane. The driving force for the transfer is the concentration gradient across the membrane. Unlike in dynamic dialysis, where two solutions continuously pass through the dialysis module, in equilibrium dialysis, at least one solution is temporarily stopped until the concentration in the acceptor solution is the same as that in the donor solution. This patented stopped-flow procedure takes about 14 min and can be directly coupled to an IC setup (Figure 2). As the dialysis is performed during the recording of the previous sample's chromatogram, the overall analysis time is not prolonged.

Figure 2

Figure 3 shows the chromatogram of a fruit yoghurt dialyzate with the following peaks: the polyol inositol, the sugar alcohol sorbitol and the mono- and disaccharides glucose, galactose, fructose, lactose, and sucrose. Repetitive analyses showed no trending in peak areas, peak heights, or retention times, which suggests that sample proteins did not pass the membrane. In contrast, carbohydrate recovery rates between 95 and 105% indicate quantitative permeability of the membrane for the target carbohydrates and thus the applicability of the dialysis technique.

Figure 3

Conclusion

High-performance anion-exchange chromatography with pulsed amperometric detection using a gold working electrode can be used to determine various carbohydrates in different foodstuff matrices. Whereas most samples require no sample preparation other than extraction, comminution, dilution or filtration, carbohydrate determination in difficult matrices can be easily solved by using Metrohm's sophisticated dialysis technique.

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